A major human rights group is accusing authorities in three Kurdish enclaves in Syria of numerous abuses, including using children in the police force.

Human Rights Watch says in a new report that the Democratic Union Party (PYD) is guilty of arbitrary arrests, denying prisoners their legal rights, and failing to investigate murders and disappearances.

The report says a new constitution in the northern enclaves of Afrin, Ain al-Arab and Jazira does ban the death penalty and guarantees some human rights, calling that a positive development.

But it says Kurdish authorities have failed to investigate the killings of nine political opponents in the last two years. The PYD has denied any responsibility for the deaths, but has not directly responded to the Human Rights Watch report.

The PYD took over governing the three Kurdish enclaves when Syrian government troops pulled out in 2012 amid the Syrian civil war.

While the predominately Kurdish areas have been quieter than war-torn parts of Syria, the PYD's armed wing has been fighting against Islamist armed groups that are trying to expand their influence.